1. Field of the Invention
The invention generally relates to an apparatus for ultrasonically measuring with high accuracy physical parameters of flowing media, more particularly the speed and volume of gas flow, by utilizing a resonance transmitter, as, for example, occurs in the measurement of gas flow in the study of respiratory function.
2. Description of the Prior Art
The operation of acoustic type flow-measuring meters type is based upon the principle that the propagation velocity of an acoustic wave in a gas flow is equal to the acoustic velocity with respect to the gas flow plus the velocity of the gas flow. Typically, such meters include a pair of acoustic transducers each adapted for both generating and detecting an acoustic pressure wave in a flow the velocity of which is to be measured. The transducers are disposed so as to define a communication link therebetween, which extends, at least obliquely, along the direction of flow. The transducers transmit an acoustic-wave packet, in turn, in alternate directions across the link while measuring the acoustic propagation period, also referred to as the acoustic time of flight, in both the upstream and the downstream directions. Finally, a difference between the upstream and downstream propagation periods is determined and provides a measure of the line integral through a velocity profile across the link of the component of gas flow in the direction of the link, usually referred to simply as the flow velocity and amount of flow.
Generally, the intensity of an ultrasound wave travelling through a medium may be attenuated by any of several different mechanisms. The ultrasonic absorption coefficient of a medium depends upon the characteristics of the medium, including its frequency characteristic. The absorption typically increases with increasing frequency, such that the high frequency components of a pulse are attenuated more than the low frequency components.
One of the most common methods by which a short ultrasonic pulse may be generated is by the application of a transient electrical pulse to the transducer. This can be done, for example, by suddenly discharging a capacitor through the transducer by switching means. As a result of the sudden application of the electrical pulse, the transducer rings at its fundamental resonant frequency. For each excitation, an exponentially decaying train of ultrasonic stress waves is radiated into the load. The ultrasonic wave amplitudes decay exponentially at a rate determined by the system Q (Quality factor).
If the wave train is sufficiently long, the decaying train of ultrasonic waves interferes with the conclusion of the propagation time due to the detection of overlapping propagated ultrasonic waves resulting from the decaying ultrasonic waves and those resulting just after the application of an exciting voltage waveform to the transducer. Some difficulty may be experienced in detecting the beginning of the propagated ultrasonic waves.